THE MANUFACTURE OF BUILDING BRICK. 225 



All the machines mentioned above are used on dry or nearly 

 drv clay, but there are several other types which are employed for 

 wet clays only, and these in addition to breaking up the clay may 

 .also; be used to mix it. The process is sometimes termed tem- 

 pering. 



Soak pits. — These are the simplest of the different types of 

 machinery used for tempering, and are employed at a number 

 of common brickyards. Their construction is simple, consisting 

 merely of a pit lined with planking and usually set immediately 

 behind the molding machine. The clay or mixture of clays, 

 with possibly some sand, is dumped into the pit, water poured 

 on, and the whole allowed to soak over night. This process 

 softens the clay, but does noi mixing, which is done entirely 

 within the molding machine. The soaked clay is shoveled 

 directly into the machine. 



Ring pits. — Ring pits (PI. XXV, Figs. 1 and 2) are employed 

 at many yards where common brick are manufactured and give 

 much better results than soak pits, for the clay receives a more 

 thorough mixing. They are of circular form, 20 to 25 feet in 

 diameter, about 3 feet deep and lined with boards or brick. Re- 

 volving in this pit is an iron wheel, 6 feet in diameter and geared 

 ■so as to i travel around the pit, and at the same time move back and 

 forth between the centre and circumference, thus thoroughly mix- 

 ing the mass. Before starting the wheel, the mixture of clays or 

 •clay and sand is dumped into 1 the pit and sufficient water added. 

 The tempering is accomplished usually in 5 or 6 hours, and one 

 pit commonly holds enough clay for 25,00.0 to 30,000 brick. 

 Two ring pits are often operated in conjunction with one mold- 

 ing machine, so that while the clay is being shoveled out of one 

 pit, the second is tempering the clay for the next day's supply. 

 Ring pits are cheaper than pug mills, but have a lower capacity 

 and require more room. They are operated by either steam or 

 liorsepower. 



Pug mills. — These are semicylindrical troughs, varying in 

 length from 3 to 14 feet, with 6 feet as a fair average. In this 

 trough there revolves a horizontal shaft, bearing knives set spirally 

 around it and having a variable pitch. The clay and water are 



15 cl G 



